Welding helmets have been used in the past to protect the eyes and face of a person doing welding (hereinafter referred to as a welder) from the very bright light occurring during welding, e.g., emanating from the welding arc, and from possible particles that may be flung toward the welder during welding. Early welding helmets had a lens through which a welder would view the work being welded and a protective shield material, such a metal, plastic or other solid material, that contained the lens and protected the welder's face from the light emitted by the welding operation and from particles. Typically the lens was a material that would transmit a relatively small amount of the incident light and, thus, when the welding was occurring would permit enough light to pass to the welder's eyes to observe the welding operation while blocking a substantial amount of the light occurring during welding so that the eyes would not be injured. Sometimes the lens was used in spectacle frames for eye protection without the face protecting shield.
Early welding helmets suffered from the disadvantage that the lenses were of fixed light transmission characteristic, i.e., darkness. Since the lenses were adequately dark (non-transmissive of or able to block transmission of some light) to perform eye protection function, it was difficult or even impossible in the absence of the welding arc to see through the lens such as to start a welding torch, arc, etc.
Efforts were made to provide variable light transmission characteristics in welding lenses and/or in welding helmets using such lenses. Several examples included variable mechanical devices, such as mechanical shutters located over the viewing area of the welding helmet to control the aperture through which light may be transmitted to the eyes of the welder. Sensors detected the occurrence of the welding light and caused a circuit automatically to close the shutter aperture.
Another early mechanical shutter used relatively rotatable polarizers in optical series. Depending on the brightness or "clearness" desired (or darkness or light attenuation desired) one polarizer was rotated relative to the other. A sensor for detecting incident light and a circuit responsive to the sensor controlled the relative rotation of the polarizers.
An example of a variable solid crystal welding lens with polarizers in protective eye glasses (spectacles) or goggles is disclosed in Marks et al. U.S. Pat. No. 3,245,315.
Other efforts made to provide variable light transmission characteristics for a welding lens have used variable liquid crystal light shutter devices. Two examples of twisted nematic liquid crystal cells used in welding lens assemblies in welding helmets are disclosed in U.S. Pat. Nos. Re. 29,684 (Gordon) and 4,039,254 (Harsch). In Gordon light shutter a twisted nematic liquid crystal cell, sandwiched between crossed polarizers, rotates the plane of polarized light received from one polarizer to pass it through the second polarizer, thus allowing a welder to see in the absence of a welding arc. In response to welding light being detected by a sensor, a circuit energizes the twisted nematic liquid crystal cell so that polarized light remains unrotated and crossed polarizers block light transmission. In Harsch minimum light transmission occurs in the deengergized (dark) state and maximum transmission occurs when the liquid crystal cells are energized (clear state). In Harsch three polarizers and two twisted nematic liquid crystal cells are arranged such that residual leakage of light through the upstream pair of polarizers and liquid crystal cell when minimum transmission is intended will be reduced by cooperation with the downstream liquid crystal cell and the further polarizer without substantially reducing transmission when in the clear (energized) state.
Alignment of three parallel directionally aligned polarizers and two twisted nematic liquid crystal cells (the five being arranged alternately in tandem to provide selective control of light transmission) also is disclosed in Fergason U.S. Pat. No. 3,918,796.
As is discussed further below, a problem encountered with prior automated welding lenses, especially liquid crystal lenses, is that they have only two operational states, a dark state and a clear state. If there were a power failure (no or too little power to the lens), the lens would fail to a predetermined state, either clear or dark. Due to characteristics of the twisted nematic liquid crystal cell, if failure were to the dark state, then speed is sacrificed; if failure were to the clear state, then eye protection upon occurrence of a power failure is sacrificed.
In such a twisted nematic liquid crystal cell, e.g., as is disclosed in U.S. Pat. Nos. Re. 29,684 and 4,039,254, nematic liquid crystal is located between a pair of generally flat plates which are pretreated at a respective surface of each by parallel rubbing or by some other process to obtain generally parallel structural alignment (alignment of the directors) of the liquid crystal material relative to the rub direction. The plates are placed in parallel to each other such that the rub direction of one surface is perpendicular to the rub direction of the other, and the liquid crystal material between the plates tends to assume a helical twist. During use, the twisted nematic liquid crystal cell is placed between a pair of plane polarizers (also referred to as linear polarizers). Light incident on the first polarizer is linearly polarized thereby and directed through the twisted nematic liquid crystal cell to the second polarizer. In the absence of an electric field input to the twisted nematic liquid crystal cell, the plane of polarization is rotated, for example, ninety degrees as the light is transmitted through the cell. Such light transmission through a twisted nematic liquid crystal cell sometimes is referred to as wave guiding of the light. In the presence of an electric field sufficient to cause alignment of substantially all of the liquid crystal material in the cell with respect to such field, the plane polarized light incident on the cell is transmitted therethrough without such rotation. Depending on the orientation of the second polarizer (also sometimes referred to as the analyzer or analyzer polarizer) relative to the first polarizer, polarized light will be transmitted or blocked, as a function of alignment of the liquid crystal material in the cell and, thus, of whether or not electric field is applied. The light transmission and the control of light transmission usually is substantially the same for any wavelength of the light.
A liquid crystal cell may be formed by a pair of generally parallel plates and liquid crystal material sandwiched therebetween, e.g., as in the several liquid crystal cells mentioned in this Background. However, it will be appreciated that a liquid crystal cell used in a welding lens or other device which embodies the principles or features of the invention alternatively may include several such liquid crystal cells in side by side relation to make up a larger area liquid crystal cell. Alternatively, other liquid crystal devices which perform functions similar and/or equivalent to those described for the liquid crystal cells herein also may be employed as a liquid crystal cell according to the invention.
Another type of liquid crystal light control device is known as a dyed liquid crystal cell. Such a dyed cell usually includes nematic liquid crystal material and a pleochroic dye that absorbs or transmits light according to orientation of the dye molecules. As the dye molecules tend to assume an alignment that is relative to the alignment of the liquid crystal structure or directors, a solution of liquid crystal material and dye placed between a pair of plates will absorb or transmit light depending on the alignment of the liquid crystal material. Thus, the absorptive characteristics of the liquid crystal device can be controlled as a function of applied electric field.
A surface mode liquid crystal cell, which is still another type of liquid crystal cell, and devices using such a cell are disclosed in U.S. Pat. Nos. 4,385,806, 4,436,376, Re. 32,521, and 4,540,243. In contrast to the wave guiding type operation of a twisted nematic liquid crystal cell, the surface mode liquid crystal cell operates on the principle of optical retardation, and, in particular, it operates to retard one of the two quadrature components (sometimes referred to as ordinary ray and extraordinary ray, respectively) of plane polarized light relative to the other. Thus, a surface mode liquid crystal cell effectively can rotate the plane of polarization of plane polarized light by an amount that is a function of a prescribed input, usually an electric field. A surface mode liquid crystal cell in effect is a variable optical retarder or variable wave plate that provides retardation as a function of the prescribed input.
Briefly, a surface mode liquid crystal cell is constructed of nematic liquid crystal (or liquid crystal that functions like nematic liquid crystal insofar as indices of refraction, birefringence, structural alignment, etc. characteristics are concerned) and a pair of generally parallel plates, each having been rubbed or otherwise treated at one surface thereof to effect desired alignment of the liquid crystal structure at the surfaces. The rub directions, for example, of the surfaces of the two plates forming the surface mode liquid crystal cell are parallel, i.e., generally in the same or opposite direction, to each other rather than at a right angle to each other (the case of the twisted nematic liquid crystal cell). Treatment also may provide for a tilt angle of the liquid crystal directors at the surfaces, as may be desired for various operational results, as is well known in the art.
The surface mode liquid crystal device may be wavelength sensitive in that the amount of retardation for a given set of conditions thereof is a function of the wavelength of the incident light. Depending on the birefringence of the liquid crystal material in a surface mode liquid crystal cell, the thickness of the layer of liquid crystal material in the cell, the magnitude of the applied prescribed input, usually electric field, when applied and the technique with which the surface mode liquid crystal cell is driven, the amount of retardation can be controlled or determined. These considerations and features are described in the above-mentioned U.S. patents related to surface mode liquid crystal cells and devices and in other literature.
In a surface mode liquid crystal cell that is positioned between a pair of plane polarizers, the amount of retardation of plane polarized light incident on the cell can be determined as a function of the aforementioned characteristics, including, as well, the wavelength of the incident light. In fact, depending on the wavelength makeup of the incident light, the effective thickness of the birefringent liquid crystal layer(s) in the cell, and the birefringence of that (those) layer(s), optical color dispersion may occur. (The principles of optical color dispersion, birefringence, optical polarization and polarized light are described in Fundamentals of Optics by Jenkins and White, McGraw-Hill Book Company, New York, 1976.) Moreover, depending on the plane of polarization of the output light transmitted through the cell and output therefrom and the relative directional orientation of the output plane polarizer (analyzer), the intensity of the transmitted light through the output plane polarizer may be varied. Therefor, if the surface mode cell is so constructed and so energized or operated that significant color dispersion does occur, then a color or wavelength filtering function can occur.
Further, a surface mode cell may be so constructed and so energized or operated that significant color dispersion does not occur. This type of operation occurs when the surface mode liquid crystal cell is operated at what is referred to as zero order, as is described in the four patents related to surface mode liquid crystal cells mentioned just above. In such operation intensity modulation of the light output from the cell can be achieved with substantial uniformity (so that there is no or only minimal distortion of the transmitted images) and without color dispersion so that the lens essentially is achromatic.
As will be described further below, the present invention is directed to a variable optical transmission controlling device which has at least three different output conditions. The device is described in detail with respect to use in a welding helmet. However, it will be appreciated that the device may be employed in other environments and in other devices and systems for controlling transmission of electromagnetic energy broadly, and, in particular, optical transmission. As used herein with respect to the preferred embodiment, optical transmission means transmission of light, i.e., electromagnetic energy that is in the visible spectrum and which also may include ultraviolet and infrared ranges. The features, concepts, and principles of the invention also may be used in connection with electromagnetic energy in other spectral ranges.
The invention is especially useful for eye protection wherein high speed protective shuttering and protective fail state are desired. Exemplary uses are in welding helmets, spectacles, goggles, and the like, as well as safety goggles for nuclear flash protection, for protection from hazards experienced by electric utility workers and for workers at furnace and electrical plant areas and at other places where bright light that could present a risk of injury may occur.
Further, the invention is described herein as undergoing certain operation in response to a prescribed input. The preferred prescribed input is an electric field. However, it will be appreciated that other prescribed inputs may be used, and reference to electric field may include equivalently such other prescribed inputs. For example, as is known, liquid crystal cells are responsive to magnetic field inputs and to thermal inputs. Other inputs also may be possible to obtain generally equivalent operation.
As is well known, the transition speed for a liquid crystal cell, whether of the twisted nematic type, dyed cell type or surface mode type, is asymmetrical; in particular, such a liquid crystal cell operates faster to achieve an operational condition, e.g., alignment of liquid crystal structure or directors, when driven to that condition by an electric field (or an increase in the field magnitude), than it operates when relaxing to a deenergized or reduced energization state, e.g., reduction or elimination of the electric field. Therefore, for maximum speed of operation to the dark state for eye protection, for example, it is desirable in a welding lens environment that the liquid crystal lens be operated with maximum power to achieve the darkest eye protection state. Also, a surface mode liquid crystal cell usually responds to energization significantly faster than a twisted nematic liquid crystal cell, and it, therefore, provides for faster operation in accordance with the present invention.
Shade number or shade is the characterization of darkness of a welding lens, for example, (hereinafter sometimes simply referred to as lens); a larger shade number represents a darker, more light blocking (or absorbing) or less optically transmissive lens and a smaller shade number represents a less dark, less light blocking (or absorbing) or more optically transmissive lens. Generally optical transmission means transmission of light and the image or view carried by the light without substantial distortion of those images, e.g., due to scattering. Shade number is a term of art often used in the field of welding and especially welding lenses for eye protection.
Clear state or clear shade means the state of highest operating luminous transmittance (or light transmission) of the lens. This state corresponds to the state having the lowest shade number for the lens.
Dark state or dark shade is the lowest operating luminous transmittance (or light transmission) of the lens. This state corresponds to the state having the highest specified shade number for the lens. The invention is described below in some instances indicating that in the dark state no light is transmitted. While this may be desirable for some applications of the principles of the invention, it will be appreciated that for a welding lens in the dark state there will be some transmission so that the welder can see to do the welding while some light is blocked to provide the desired eye protection from damage, injury or the like by the light emitted during welding.
Intermediate state or intermediate shade is one that is not the clear state and the dark state. It may be between the clear state and the dark state, but this is not always necessary, for it may even be darker than the dark state if such operation could be achieved. According to the described embodiment of the invention, the intermediate state provides an intermediate level of light transmission/absorption, i.e., between the clear state and the dark state, and occurs during power failure, inadequate power, or power off status of the lens.
Power failure, failure, or fail state means absence of power being delivered to the lens. Power failure also may mean that inadequate power is being delivered to the lens to cause it to assume the desired state.
Off state is the condition of the lens when no electrical power is being supplied to the lens; the off state also is referred to as the power-off state. As is described below, the intermediate shade preferably occurs in the welding lens during the off state. The fail state of the lens also is the off state, i.e., no power is provided due to failure of the power supply; and the fail state also may occur when there simply is inadequate power available to drive or to energize the lens to the clear state and dark state.
Shutter response time is the time required for the circuitry associated with the lens to detect a sharp increase in incident light (e.g., due to striking of the welding arc, etc.) and to switch the lens from the clear state to the dark state.
Shutter recovery time is the time required for the circuitry associated with the lens to detect a sharp decrease in light (e.g., due to extinguishing of the welding arc, etc.) and to switch the lens from the dark state to the clear state.
Variable transmittance is the ability of the lens to be switched from one level of luminous transmittance (also referred to as transmission of light) to another level of luminous transmittance in response to a change in incident illumination.
Dynamic operational range or dynamic optical range is the operational range of the lens between the dark state and the clear state, e.g., the difference between the shade numbers of the dark state and the clear state.
As was mentioned above, a problem encountered with prior automated welding lenses, especially liquid crystal lenses, is that they have only two operational states, a dark state and a clear state. If there were a power failure (no or too little power to the lens), the lens would fail to a predetermined state, either clear or dark. If failure were to the dark state, then speed is sacrificed; if failure were to the clear state, then protection upon occurrence of a power failure is sacrificed.
More specifically, it is known that alignment of liquid crystal material (or the directors thereof) occurs faster when energy is applied or is increased from a lower energization level compared to slower alignment due to relaxation or the like when energy is removed or reduced. Therefore, if the conventional twisted nematic liquid crystal lens, upon failure of the power supply, would fail to a dark state, such dark state would be the result of "relaxation" of the liquid crystal. Accordingly, the operation of the lens would be relatively slow in response to the commencing of welding and the bright light produced thereby, thus minimizing the protection for the welder. If failure were to the clear state, then the occurrence of power failure during (or at the inception of) welding would result in the lack of protection for the welder. Further, if two liquid crystal cells were arranged in optical series with appropriate polarizers, e.g., as in the above-mentioned Harsch patent, an intermediate darkness failure mode could be made possible by orienting one of the analyzer polarizers to transmit the plane polarized light received from its immediately optically upstream deenergized twisted nematic liquid crystal cell and the other analyzer polarizer to block the plane polarized light received from its immediately optically upstream twisted nematic liquid crystal cell. Then, for a dark state, one liquid crystal cell is energized and the other is deenergized; and for the clear state the opposite liquid crystal cell is energized and the other is deenergized. In this case a power failure would allow reduced light transmission due to the dark state of one of the twisted nematic liquid crystal cells (with its pair of polarizers), but the overall lens would suffer in reduced speed due to the needed relaxation time for the other liquid crystal cell when the lens is switched from clear state to dark state in order to achieve maximum darkness of the lens in response to inception of welding.
It would be desirable to provide an improved variable light transmission controlling device, especially one for use in welding helmets or the like, which provides both a clear state and a dark state as well as an intermediate state which in a failure mode, such as in the absence or inadequacy of power to the device, is darker than the clear state, e.g., so as to provide a temporary measure of eye protection and to minimize risk, or even is as dark or darker than the dark state. It would be further desirable to provide such characteristics while also providing fast operational speed for the light transmission controlling device in switching to the dark state, for example, by providing switching to the dark state as a powered or driven condition of the lens.
The entire disclosures of all of the above-mentioned patents and patent applications are hereby incorporated by reference.